Remove dead pread and pwrite replacement code.

pread() and pwrite() are in SUSv2, and all targeted Unix systems have
them.

Previously, we defined pg_pread and pg_pwrite to emulate these function
with lseek() on old Unixen.  The names with a pg_ prefix were a reminder
of a portability hazard: they might change the current file position.
That hazard is gone, so we can drop the prefixes.

Since the remaining replacement code is Windows-only, move it into
src/port/win32p{read,write}.c, and move the declarations into
src/include/port/win32_port.h.

No need for vestigial HAVE_PREAD, HAVE_PWRITE macros as they were only
used for declarations in port.h which have now moved into win32_port.h.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Greg Stark <stark@mit.edu>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com

src/port/preadv.c

@@ -30,7 +30,7 @@ pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 {
 #ifdef HAVE_READV
 	if (iovcnt == 1)
-		return pg_pread(fd, iov[0].iov_base, iov[0].iov_len, offset);
+		return pread(fd, iov[0].iov_base, iov[0].iov_len, offset);
 	if (lseek(fd, offset, SEEK_SET) < 0)
 		return -1;
 	return readv(fd, iov, iovcnt);
@@ -40,7 +40,7 @@ pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 
 	for (int i = 0; i < iovcnt; ++i)
 	{
-		part = pg_pread(fd, iov[i].iov_base, iov[i].iov_len, offset);
+		part = pread(fd, iov[i].iov_base, iov[i].iov_len, offset);
 		if (part < 0)
 		{
 			if (i == 0)

src/port/pwritev.c

@@ -30,7 +30,7 @@ pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 {
 #ifdef HAVE_WRITEV
 	if (iovcnt == 1)
-		return pg_pwrite(fd, iov[0].iov_base, iov[0].iov_len, offset);
+		return pwrite(fd, iov[0].iov_base, iov[0].iov_len, offset);
 	if (lseek(fd, offset, SEEK_SET) < 0)
 		return -1;
 	return writev(fd, iov, iovcnt);
@@ -40,7 +40,7 @@ pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 
 	for (int i = 0; i < iovcnt; ++i)
 	{
-		part = pg_pwrite(fd, iov[i].iov_base, iov[i].iov_len, offset);
+		part = pwrite(fd, iov[i].iov_base, iov[i].iov_len, offset);
 		if (part < 0)
 		{
 			if (i == 0)

src/port/win32pread.c

@@ -1,15 +1,12 @@
 /*-------------------------------------------------------------------------
  *
- * pread.c
- *	  Implementation of pread(2) for platforms that lack one.
+ * win32pread.c
+ *	  Implementation of pread(2) for Windows.
  *
  * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  src/port/pread.c
- *
- * Note that this implementation changes the current file position, unlike
- * the POSIX function, so we use the name pg_pread().
+ *	  src/port/win32pread.c
  *
  *-------------------------------------------------------------------------
  */
@@ -17,16 +14,11 @@
 
 #include "postgres.h"
 
-#ifdef WIN32
 #include <windows.h>
-#else
-#include <unistd.h>
-#endif
 
 ssize_t
-pg_pread(int fd, void *buf, size_t size, off_t offset)
+pread(int fd, void *buf, size_t size, off_t offset)
 {
-#ifdef WIN32
 	OVERLAPPED	overlapped = {0};
 	HANDLE		handle;
 	DWORD		result;
@@ -49,10 +41,4 @@ pg_pread(int fd, void *buf, size_t size, off_t offset)
 	}
 
 	return result;
-#else
-	if (lseek(fd, offset, SEEK_SET) < 0)
-		return -1;
-
-	return read(fd, buf, size);
-#endif
 }

src/port/win32pwrite.c

@@ -1,15 +1,12 @@
 /*-------------------------------------------------------------------------
  *
- * pwrite.c
- *	  Implementation of pwrite(2) for platforms that lack one.
+ * win32pwrite.c
+ *	  Implementation of pwrite(2) for Windows.
  *
  * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  src/port/pwrite.c
- *
- * Note that this implementation changes the current file position, unlike
- * the POSIX function, so we use the name pg_pwrite().
+ *	  src/port/win32pwrite.c
  *
  *-------------------------------------------------------------------------
  */
@@ -17,16 +14,11 @@
 
 #include "postgres.h"
 
-#ifdef WIN32
 #include <windows.h>
-#else
-#include <unistd.h>
-#endif
 
 ssize_t
-pg_pwrite(int fd, const void *buf, size_t size, off_t offset)
+pwrite(int fd, const void *buf, size_t size, off_t offset)
 {
-#ifdef WIN32
 	OVERLAPPED	overlapped = {0};
 	HANDLE		handle;
 	DWORD		result;
@@ -46,10 +38,4 @@ pg_pwrite(int fd, const void *buf, size_t size, off_t offset)
 	}
 
 	return result;
-#else
-	if (lseek(fd, offset, SEEK_SET) < 0)
-		return -1;
-
-	return write(fd, buf, size);
-#endif
 }